This relates generally to imaging devices, and more particularly, to image sensors that include backside illuminated global shutter imaging sensor pixels.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. In a typical arrangement, an electronic device is provided with an array of image pixels arranged in pixel rows and pixel columns. Each image pixel in the array includes a photodiode that is coupled to a floating diffusion region via a transfer gate. Column circuitry is coupled to each pixel column for reading out pixel signals from the image pixels. The column circuitry often implements a correlated double sampling (CDS) procedure, which involves obtaining pixel signals by computing the difference between reset signals sampled during reset operations and image signals sampled following charge transfer operations.
In a configuration that includes frontside illuminated image pixels, a dielectric stack is formed on the front surface of a semiconductor substrate directly on top of the photodiodes. The dielectric stack includes metal routing lines and metal vias formed in dielectric material. Image light passes through the dielectric stack to the photodiodes. In a configuration that includes backside illuminated image pixels, the photodiodes receive image light through the backside of the semiconductor substrate so that the light does not pass through the dielectric stack.
Image sensors can operate using a global shutter or a rolling shutter scheme. In a global shutter, every pixel in the image sensor may simultaneously capture an image, whereas in a rolling shutter each row of pixels may sequentially capture an image. Pixels in an image sensor that operates in global shutter mode may each include a photodiode for detecting incoming light and a separate storage diode for temporarily storing charge. If care is not taken, parasitic light may be scattered or diffracted into the storage diode, lowering the global shutter pixel efficiency and leading to image artifacts such as vertical shading and moving object smear.
To shield the storage diodes from parasitic light, conventional global shutter pixels may be frontside illuminated pixels with shielding layers in the dielectric stack above the storage diodes. However, frontside illuminated pixels may have limited performance compared to backside illuminated pixels.
It would therefore be desirable to provide improved backside illuminated global shutter imaging pixels for image sensors.